A fuel processor converts a fuel, typically a hydrocarbon (and including related materials such as alcohols) into a mixture of hydrogen (H2), carbon dioxide (CO2), and often other components. The key reaction steps are dissociating the fuel in the presence of water (as steam) and heat, typically at 700 to 1000 deg. C., into a mixture of carbon monoxide (CO) and H; using the “water gas shift” to convert the CO2 into CO and H2 by reaction with steam; and subsequent cleanup steps, where required, to remove trace materials which would inhibit the fuel cell reaction. The resulting hydrogen-containing gas is usually referred to as “reformate”.
The reformate is supplied to the anode of a fuel cell, and oxygen or air is supplied to the cathode. The electrodes are separated by a semipermeable barrier that passes only selected components of water (typically H+ or OH− or O2−), while electrons travel through an external circuit as an electric current. Various types of fuel cells are known. The systems of the invention are particularly adapted to a PEM (polymer electrolyte membrane) type of fuel cell, in which the membrane passes only H+ ions. Current versions of such membranes require humidification to operate, and are limited to temperatures below about 100 deg. C. However, many of the attributes of the systems of the invention will be applicable to PEM cells operating at higher temperatures, or to other low-temperature (for instance, less than about 200 deg. C.) fuel cell types.
It has been proposed that a small fuel cell, optionally with a small reformer if hydrogen is not available, could be integrated with a building or a group of buildings to use both the electricity and the waste heat of either the fuel cell or the reformer to provide space heating and/or water heating, as well as locally generated electricity. This is known as cogeneration. Generally, the electricity will be used in building(s), or exported to an electric grid, and the waste heat of the fuel cell will be used to provide space heating and/or hot water. Reformer waste heat is in some cases also considered for heating use. Thus, the “fuel cell power system”, or FCPS, comprising a reformer, a fuel cell, and ancillary apparatus, “co-generates” both electricity and heat for use at a site.
U.S. Pat. No. 5,985,474 to Chen et al. discloses an integrated system which includes a fuel cell assembly for supplying electrical power to a building, a furnace having a heating chamber and a heat exchanger for supplying heat to the building, and a reformer for providing a supply of reformate directly to the furnace and the fuel cell assembly.
U.S. Pat. No. 6,013,385 to DuBose discloses a fuel cell gas management system including a cathode humidification system for transferring latent and sensible heat from an exhaust stream to the cathode inlet stream of the fuel cell; an anode humidity retention system for maintaining the total enthalpy of the anode stream exiting the fuel cell equal to the total enthalpy of the anode inlet stream; and a cooling water management system having segregated deionized water and cooling water loops interconnected by means of a brazed plate heat exchanger.
U.S. Pat. No. 6,290,142 to Togawa et al. discloses a cogeneration apparatus arranged to properly respond to a plurality of separate demands for supplying the thermal energy. A controller controls the operation of the engine generator in response to the conditions of thermal loads and determined by the measurements of temperature detected by the temperature sensors TS1 and TS2.
U.S. Pat. No. 5,335,628 to Dunbar discloses a fuel cell and a boiler coupled in such a manner that the water used to capture excess heat generated by the fuel cell is used for boiler feedwater heating. In one embodiment, steam generated by the boiler is used in an operation that converts the steam to condensate, and the condensate is returned to the fuel cell for use as a heat sink for the thermal energy generated within the fuel cell unit.
U.S. Pat. No. 5,969,435 to Wilhelm discloses a DC power system which receives AC electrical power and DC electrical power from separate first and second sources simultaneously. The DC power system delivers DC electrical power to an output for use by a load requiring DC power. The DC power system includes a converter to convert AC electrical power to DC electrical power and a power sharing control device to control and distribute the DC electrical power to an output.
The present invention distinguishes over prior art attempts by providing an efficient and effective cogeneration system. In particular, the prior art fails to teach where in the process heat is available for reclamation, and how such heat can be used without significantly decreasing either the efficiency or the reliability of a fuel cell/fuel processor system. These issues are far from trivial. There are many potential sources of heat in the system, including but not limited to the burner or burners, the reformer, the water-gas-shift reactors, a preferential oxidation (cleanup) reactor, the fuel cell stack, and various heat exchange fluids which may be shared among these components. Such fluids include, but are not limited to, water, steam, air, exhaust, fuel and reformate. It is not self-evident which sources can be used to provide heating value without disturbing the efficiency of the other components or the stability of the operating system. Moreover, the various components of the system operate at a variety of different temperatures, ranging from as high as about 700 to 1000 deg. C. in reforming, (or even higher in a burner), down to around 50 deg. C. (reformate entering a PEM stack), and even further down to around 20 deg. C. (water entering the system).
The present inventors have investigated and solved these problems by designing, and constructing or simulating, various potential cogeneration systems. Improved interconnections and devices for extracting heat from a FCPS (fuel processor/fuel cell system) have been discovered which allow the FCPS to operate at least as efficiently as similar non-cogeneration systems. Moreover, the systems of the present invention are typically simple to operate, with a tendency to self-regulate. Additionally, they can recover water efficiently and can easily be integrated with a conventional furnace or boiler used for heating. Therefore, the present invention is provided to solve the problems discussed above and other problems.